Anthony R. LupoDepartment of Soil, Environmental, and Atmospheric

Science 302 E ABNR building

University of Missouri – ColumbiaColumbia, MO 65211

Энтони Р. ЛупоФакультет: почвеные, естественные, и

атмоферные науки302 Е АВЕ здание

Университет МиуссурииКолумбя, МО 65211

Atmospheric blocking is a phenomenon that may occur relatively rarely (compared to smaller scale phenomenon), but may dominate a region’s weather for a whole season.

Summer 2010 was a prime example as blocking dominated Russia’s weather from May to mid-August.

There is not even a set definition, or criterion that is commonly accepted in meteorology today.

A generic definition may be: a long-lived, mid-latitude, mid-tropospheric positive geopotential anomaly.

The basic climatological characteristics of blocking have been well known since the 1950’s (e.g., Rex, 1950). That is, blocking is generally a winter season feature that occurs downstream of the storm tracks over the oceans.

This presentation will focus on what new contributions were made by this research group.

Blocking is still not a well – forecast problem, especially at onset and termination. These processes are still not well understood.

Blocking can be responsible for economic (e.g. agricultural, capital, and infrastructure) and societal losses (e.g. cost of human life) , directly and indirectly.

The work of Lupo and Smith (1995a) demonstrated that all blocking events are accompanied by a strong developing cyclone.

They did not propose the idea of upstream, precursor cyclones (e.g., Kalnay, 1979; Frederiksen, 1982; Colucci,1985; Mullen 1986,1987; Agayan and Mokhov, 1989), but demonstrated that these were present in every case, and in maintenance.

They also demonstrated that there was a correlation between the vigour of mid-latitude cyclone development and block intensity and duration.

Agayan and Mokhov (1989) was the first to demonstrate that blocking was an important contribution to Northern Hemisphere continental climate regimes. Lupo and Smith (1995) used this idea to construct a separate climatological category.

Lupo and Smith (1995) developed the first intensity index for blocking and based this on the strength of mid-latitude height gradients. This index was refined by Wiedenmann et al. (2002) [Lupo, Mokhov, Tikhonov].

Lupo, Oglesby, and Mokhov (1997) were the first to propose that blocking characteristics may change with climate (become more persistent, occur more often, but weaker overall)

In a combined paper (Wiedenmann et al. 2002) our research was the first to demonstrate that the occurrence and strength of blocking could be correlated to interannual climate variations.

We showed that blocking was more frequent and intense during La Niña (El Niño) years in the Northern (Southern) Hemisphere. This result could be linked to cyclone variability.

We also demonstrated that up until 1999, the long term trend in the SH was for fewer blocking events, and this could be related to the increasingly zonal SH flow in the mid-latitudes.

New analysis (Lupo et al. 2008) suggests blocking to be on the increase again globally and this may be related to the Pacific Decadal Oscillation in the SH, or climate change.

Early views of blocking recognized it to be the result of large-scale waves becoming quasi-stationary due to resonance with topographical features.

Some studies, such as Charney and Devore (1979), for example, considered that blocking may be a distinct atmospheric flow regime (energy state) different from that of non-blocked flow.

Regardless, blocking was thought to be the result of hemispheric-wide flow processes.

The contribution of synoptic-scale cyclones was speculated by early pioneers such as Frederiksen, Shutts, or Kalnay and Merkine. These studies used analytical or numerical models.

Nonetheless, these earlier works suggested that the storm track (small-scale cyclones) would support block formation so long as the cyclones were upstream of the block.

In the 1980s, work by Colucci and Mullen suggested that as long as the cyclone wave was about ¼ wavelength (90o out of phase) upstream, the blocking event would form.

This view point of blocking suggests that local forcing was the key to the development of blocking events, not hemispheric-wide forcing.

This hypothesis was supported by the statistical analysis of Legenas and Okland (1983) who suggested that if blocking were forced by hemisphere –wide, there should be a greater occurrence of simultaneously occurring events.

Tsou and Smith (1991) used the height tendency equation to examine the growth of individual blocking events in a similar manner to that of mid-latitude cyclones.

Lupo and Smith (1995) then used a modified form of the vorticity equation to examine what processes contribute to block formation, maintenance and decay.

•As in the study of cyclones, these techniques treat blocking events as distinct events.

•The type of equations used combines the Vorticity Equation with the First Lawof Thermodynamics.

The key result of this study was to demonstrate that the amplification of the synoptic-scale wave strengthens the anticyclonic vorticity field and transport into the blocked region.

This occurred not only during block formation, but helped to maintain it.

They also showed that the transport of cold air in the lower troposphere into the anticyclone region by a synoptic-scale transient contributed to the decay of the event.

This was the first study to suggest upstream transients could also force decay.

This group then turned their attention to the Southern Hemisphere.

Why are there so few events there?

Why do they translate more than in the NH?

Burkhardt and Lupo (2005) examine SH blocking, and in particular look at the interactions using the Potential Vorticity equation:

They find that blocking in the SH results primarily from the superposition of large and small scale waves rather than interaction as in NH.

None of these studies has solved the problem of poor forecasts for the development and decay of blocking.

Thus, this group (Lupo – Dostoglou – joins Mokhov again! - Izvestiya) utilized the “Dymnikov conjecture” to examine flow stability.

This relationship should be valid for large-scale flows that are non-divergent and non- deformational.

The main results of this work demonstrated that blocking, once formed, can be relatively better predicted, but that the large-scale flow can become unstable.

They (Lupo, Mokhov, Dostoglou) also show that blocking will decay under four scenarios

1. The active role of the synoptic transients as suggested by Bosart and Lupo (1999)

2. No positive or negative support from the synoptic scale (Bosart and Lupo, 1999)

3. and 4. Same as 1and 2, except with abrupt change in the large scale.

Hussain and Lupo (2010) examined 126 NH blocking events over a three-year (2002-2004) period using the Dymnikov relationship. In every case, the development and decay were associated with abrupt changes in area averaged enstropy.

The area integrated enstrophy may have value as a tool for predicting the formation of blocking.

The research efforts of both our groups has led to advances in the understanding of the climatological and dynamic character of blocking.

In spite of these advances, several questions remain including the forecasting of development and decay, and how the nature of blocking may change as climate changes.

Class of 1988! (Rules?)

Questions?  Comments?  Criticisms?

Email: lupoa@missouri.edu